With the advent of a resist for KrF excimer laser (248 nm), an image forming method called chemical amplification is used as the image forming method for a resist so as to compensate for the reduction in sensitivity due to light absorption. To explain the image-forming method of positive chemical amplification by example, this is an image forming method of causing an acid generator in the exposed area to decompose upon exposure and thereby generate an acid, utilizing the generated acid as the reactive catalyst to change an alkali-insoluble group to an alkali-soluble group by baking after exposure (PEB: Post Exposure Bake), and removing the exposed area by alkali development.
Along with miniaturization of a semiconductor device, the trend is moving into a shorter wavelength of the exposure light source and a higher numerical aperture (higher NA) of the projection lens and at the present time, an exposure machine using, as its light source, an ArF excimer laser having a wavelength at 193 nm is developed. Also, a so-called immersion method of filling a high refractive-index liquid (hereinafter sometimes referred to as an “immersion liquid”) between the projection lens and the sample is being aggressively studied. The immersion method can be combined with the super-resolution technology under study at present, such as phase-shift method and modified illumination method.
As for the technique to more enhance the resolution, double exposure technology or double patterning technology has been proposed.
In conventional pattern formation of an electronic device such as semiconductor device, a mask or reticle pattern enlarged in pattern size by 4 to 5 times as compared with a pattern intended to form is reduced and transferred on an exposure target such as wafer by using a reduction projection exposure apparatus.
However, the dimensional miniaturization brings about a problem that in the conventional exposure system, lights irradiated on adjacent patterns interfere with each other to decrease the optical contrast. Therefore, in such technology, it is devised to divide the exposure mask design into two or more designs and synthesize an image by independently exposing these masks. In this double exposure system, it is necessary to divide the exposure mask design and again synthesize an image on an exposure target (wafer), and therefore, the division of the mask design must be devised so that the pattern on the reticle can be faithfully reproduced on the exposure target.
Studies of applying the effect of such a double exposure system to the transfer of a fine image pattern of a semiconductor device are introduced, for example, in JPA-2006-156422 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”).
In the case of simply applying the conventional resist composition to the double exposure system, the pattern formation needs to be performed in the vicinity of resolution limit of the resist, and this incurs a problem that sufficient exposure margin or depth of focus cannot be obtained.
JP-A-2000-199953 describes a double development technology as the double patterning technology for enhancing the resolution. In this case, an image forming method by general chemical amplification is utilized, and by making use of a property that the polarity of a resin in a resist composition when exposed is changed to high polarity in a region irradiated with a high light intensity and maintained as low polarity in a region irradiated with a low light intensity, the high exposure region of a specific resist film is dissolved with a high-polarity developer (specifically, a conventional aqueous alkali solution) (positive development) and the low exposure region is dissolved with a low-polarity developer (specifically, an organic solvent) (negative development). More specifically, the region not lower than an exposure dose E2 of irradiation light 1 shown in FIG. 1 is dissolved using an aqueous alkali solution, and the region not higher than an exposure dose E1 is dissolved using a specific organic solvent, whereby, as shown in FIG. 1, the region with a medium exposure dose (E2−E1) is allowed to remain without being developed and an L/S pattern 3 having a pitch half the pitch of the exposure mask 2 is formed on a wafer 4.
However, it is very difficult to select an optimal combination of a resist composition and an organic solvent developer, and there is a problem that developability when using an organic solvent developer becomes bad.
Furthermore, at the formation of a fine pattern by double development, good resolution achievable when merely using an organic solvent or an aqueous alkali solution alone is insufficient, and the resist film is demanded to show good pattern resolution for both developers.
Considering these problems, in the double development technology, U.S. Patent Application Publication 2008/0187860 proposes a pattern forming method using a specific resist composition. According to this technique, a high-precision fine pattern is supposed to be stably obtained.
Yet it is required that a higher-precision fine pattern excellent in terms of line width variation (LWR) and focus latitude (DOF) is stably obtained.